1. Field of the Invention
The present invention relates to spindles and, in particular, to a spindle for gripping and closing a mounting member.
2. Description of the Related Art
Spindles are widely used for providing rotary motion to workpieces and/or tools in many applications, including tool making, machining, and general industrial production. Ordinarily, a mounting member such as a slotted collet or the like will be provided for mounting the workpiece or tool to be rotated on the spindle. The spindle must, therefore, include some provision for gripping and closing the mounting member to hold the workpiece or tool, and this requirement is typically met by providing a drawbar threaded to engage the mounting member or collet. The drawbar can then be moved along its axis to draw the collet against a tapered contacting surface and thereby contract the slotted collet around the workpiece or tool to hold it securely.
In order to improve the performance of spindles, fluid bearings, which are also known as hydrostatic bearings, have been used to an extent to enable higher maximum RPMs and to increase reliability. Fluid thrust bearings provide high dynamic stability and accuracy in applications where axial forces are present. In addition, fluid bearings may permit the use of water and other advantageous fluids as lubricants, which reduces the environmental problems and fire hazards created through use of typical oil-type lubricants.
Heretofore known spindles with fluid thrust bearings have, however, not included satisfactory capability for drawbar-operated closing of mounting members as described above, which requires that the drawbar be movable axially so as to draw the mounting member against the tapered contacting surface. The combined capability of rotary and axial movement has proven difficult to advantageously achieve in spindles with fluid bearings.
Moreover, many known spindles are driven by a drive-belt arrangement in which the drive motor is positioned remote from the spindle. Such an arrangement may have disadvantages in that the belt adds additional inertial drag to the spindle, thereby reducing the ability of the spindle to accelerate quickly to a desired speed, or to decelerate when desired. Moreover, failure of the belt will, of course, render the spindle inoperable until repairs can be made, and the maximum speed capacity of the belt also limits the overall performance of the spindle. Integral drive motors, which drive a spindle or shaft directly without the need for a belt to transmit driving force, provide many advantages over a drive-belt arrangement, including quicker acceleration, faster response to control inputs, and the device driven by the motor. The advantages of integral drive motors, which eliminate drive belts, have not been fully realized in a spindle having a fluid bearing.
Thus, a need exists for a spindle which includes a fluid thrust bearing which can also grip and close a mounting member, thereby increasing the performance of the spindle and also allowing it to be used in typical applications requiring a workpiece or a tool to be mounted in the mounting member. In addition, a need exists for such a spindle which includes an integral drive motor.